1. Technical Field
This invention relates generally to storage tanks for hot fluids and, more particularly, to dip tubes for hot water storage tanks such as water heaters.
2. Background Art
There has been a lingering problem commonly associated with direct-fired gas and oil water heaters having an upright storage tank in which the center gas flue is surrounded by the water stored within the tank. Frequent withdrawals of small quantities of hot water causes frequent firing of the gas/oil burner. The gas/oil burner is controlled by a thermostat located in the bottom of the tank. With each draw of hot water, the burner is turned on to heat the newly introduced cold water. At the same time, the previously heated water has moved up in the tank and gains temperature due to heat transfer from the flue. This phenomenon continues as the water rises within the tank so that the temperature of the water in the dome of the tank reaches extremely high temperature. This high temperature water may create an extreme safety risk. In addition, the safety relief valve located in the tank may be opened to drain hot water from the tank. Operation of the valve in such a manner would waste energy by draining heated water.
This phenomenon of hot water accumulating near the tank dome by the aforementioned process is sometimes referred to as "stacking". Since the thermostat is located in the bottom of the heater, the temperature of the water in the top can be dangerously high. To alleviate this problem, present water heaters use dip tubes which terminate above the bottom of the tank about one third of the way up. Some heaters use a dip tube design which partially introduces incoming cold water in the upper portion of the tank to moderate the excessively high water temperature. To accomplish this, the dip tube is provided with a hole which opens or closes in response to the temperature of the surrounding water. Both these solutions have an adverse effect on the recovery rating of the heater. The invention disclosed herein can provide a solution to the "stacking" problem without adversely affecting the recovery rate.
The shorter dip tube has a second disadvantage. It keeps the water in the tank bottom relatively stagnant. This allows the mineral sediment to settle to the bottom of the tank undisturbed. Over time, these sediments can accumulate and reduce the useful gallon capacity of the tank. Also, the sediments reduce the heat transfer from the burner to the water through the bottom. Since the tank bottom is the primary heat transfer area for the tank's burner, this reduces heat transfer efficiency which, in turn, reduces energy efficiency. This also exacerbates the stacking problem, because more heat is transferred through the stack rather than at the bottom. Accumulation of sediments in the bottom also creates severe stresses on the tank causing premature tank failure. Further, the sediments block the drain valve sometimes making it inoperative or ineffective. All of these problems can be alleviated or curtailed substantially if the dip tube is brought all the way down in the bottom and the incoming water jet is used to keep sediments in motion and in suspension so that the sediments will eventually flow out the tank.
Some water heaters have appeared on market which approach this problem in a different manner. In those heaters, the water tank is placed inside another tank which is filled with a captive liquid heating medium. The burner heat is transferred to the liquid heating medium in the outer tank, the liquid medium being kept in circulation. This liquid medium transfers heat to the water. This is extremely expensive since two tanks and an electric motor to circulate the liquid medium and its associated controls are required for operation. The circulating medium normalizes the temperature of the water thereby reducing the recovery rating.
For a moderate cost, the heat exchanger dip tube proposed here will provide a solution to the above-stated problems caused by the build up of sediments, namely, loss of heat transfer (loss of energy efficiency), reduction in the tank's original gallon capacity, and increased safety hazard.